1. Field of the Invention
The present invention relates to a fuel injector.
2. Description of the Prior Art
Fuel injectors of the type of interest here serve to control the fuel that is injected into the combustion chamber in an internal combustion engine. They are constructed essentially of a magnet valve and a miniature servo valve, and they actuate a nozzle needle the opening and closing position of which is controllable by the magnet valve, so that injection bores in the injector are opened and closed for injection of the fuel.
A fuel injector of this kind is known from German Patent Disclosure DE 101 59 003 A1. In it, a fuel injector is disclosed which is embodied with a magnet valve for controlling the miniature servo valve, with an armature that can be placed in a valve seat in the lower armature chamber. The lower armature chamber communicates fluidically via bores with a control pressure chamber, and leakage quantities that occur via at least one return bore can be returned to a tank via the lower armature chamber. Upon closure of the valve seat by the armature, in order to avoid pressure fluctuations in the system of return bores below the valve seat, means are provided in the lower armature chamber for reducing these pressure fluctuations. The means for reducing pressure fluctuations include recesses to be machined in the lower armature chamber or fixtures in it, as well as increased volume of the return bores or of the lower armature chamber. Thus certain portions in both the magnet valve and the injection valve that are affected by the return of the leakage quantities can be embodied with enlarged volumes. Such an enlargement of the volume, with a defined outflow cross section, markedly reduces pressure fluctuations, but the requisite volume near the injector for the purpose is not available in the existing installation space.
Such pressure fluctuations are a substantial disadvantage of the known versions of fuel injectors; they can lead to a variable opening performance of the miniature servo valve and hence to fluctuations in the quantity of fuel injected. Pressure fluctuations that spread via connecting bores into the adjoining magnet valve chamber and magnet spring chamber cause waviness of the characteristic quantity curve, which cannot be reduced or avoided satisfactorily even by means of the enlarged fluidic volumes. Moreover, a high pressure level in the fuel return causes impermissibly high stresses in fuel return hoses or increased costs for high-pressure-proof hoses. In the leakage bore in the injector body, cavitation damage occurs, which is caused by pressure fluctuations and high flow speeds.
To avoid excessive pressures and the recoil behavior of the armature that impedes clean closing of the injection ports by the nozzle needle, an unclean injection of the fuel in the concluding phase of the opening cycle is brought about, but that causes poor emissions of the internal combustion engine.
From German Patent Disclosure DE 102 21 383 A1, pressure limiting devices for limiting peak pressure values occurring in the fluidic system of a fuel injector are known. These devices relate to a fuel injector which has a high-pressure fuel pump with a pump piston, which is driven in a reciprocating motion and defines a pump work chamber that communicates with at least one fuel injector, by which fuel is injected into the combustion chamber of the engine. Here, by an electrically actuated control valve, at least one connection of the pump work chamber with a relief region is controlled. By the pressure limiting device, if a predetermined pressure is exceeded in the pump work chamber, a connection of the pump work chamber with a relief region is opened up. The pressure limiting device has an elastically deformable diaphragm, which is acted upon by the pressure prevailing in the pump work chamber and which by its elastic deformation, if the predetermined pressure in the pump work chamber is exceeded, opens the connection of the pump work chamber with the relief region.
However, a disadvantage of the proposed pressure limiting device is the outflow of fuel into a relief region, which does not make it possible to make a closed system, or in other words to integrate the pressure limiting device with the closed fluidic system of return bores, without a leakage flow.